The present application relates to an improved process for manufacturing articles of foamed thermoplastic material and an article produced thereby, and more particularly, wherein foaming of the thermoplastic material is produced by means of a foaming agent mixed therein.
It is well known to manufacture articles having a cellular structure of uniform weight and minimum density from a thermoplastic material by injection molding. Such articles, particularly as used in the footwear industry, are generally formed of polyethylene, polypropylene, polystyrene, or thermoplastic rubber, through the use of rotary type injection molding machines which have a plurality of injection stations.
However, the cellular structure of the foamed thermoplastic material exhibits the characteristic of inhibiting heat dispersion from the interior of the manufactured article to the mold. As a result, molding times are disadvantageously extended, thereby slowing down the rate of production of the intended machines, i.e., the molding machines cannot operate at their optimum production rate.
Another disadvantage associated with conventional injection molding processes is the formation of blowholes or bubbles on the outer surface of the manufactured article, particularly when low density polyethylene is utilized, as a result of excess gas being entrapped between the manufactured article and the mold. Attempts which have been made to correct this problem by the addition to the injected material of such agents as inoculants having particular characteristics of fluidity and density, or through utilizing suitable filter inserts for exhausting the excess gas exteriorly of the mold, have given very poor results which have not been satisfactory.
A further, but not last, disadvantage of conventional injection processes is that a drop of foamed material is formed on the inlet conduit (sprue bushing) of the mold, which drop often has to be manually removed prior to the next injection cycle.
In view of the foregoing, it is a principal object of the present invention to provide an improved process for creating articles manufactured from foamed thermoplastic materials which will avoid the above-noted disadvantages of the prior art processes, particularly by substantially reducing molding cycle times.
In accordance with a preferred embodiment of the present invention, the above-noted object is achieved through an injection molding process wherein cold compressed gas, e.g. air, is injected into the cavity of the mold internally of the mass of injected thermoplastic material contained in said cavity, while the material is still in its molten or plastic state.
In this regard, it is noted that the introduction of air into a mass of material contained within a mold is not novel per se. For example, in U.S. Pat. Nos. 4,106,887 and 4,129,635 to Yasuike, et al. a method and apparatus are disclosed wherein a gas under pressure is injected into a still molten material within a mold cavity so as to create a hollow within the foamed material. However, formation of a void within the article produced in accordance with the present method is a by product, not a sought after object. Reflecting this difference in purpose, Yasuike, et al. teach heating of their gas prior to its being charged into the mold cavity so as to prevent the resin material molded by them from cooling, in contrast to the present method which deliberately injects cold gas into the molten material so as to cool the same.
Similarly, the use of internal air cooling in connection with the production of articles of expanded material is likewise known per se. For example, in Hanning U.S. Pat. Nos. 4,115,491 and 4,033,710, a method and apparatus is disclosed wherein hollow articles of foamed material are produced in a mold into which a tube is extended for the admission of a coolant into the cavity designed to expedite the hardening of the thermoplastic material therein. However, again, the Hanning patents have for a basic object the obtaining of a desired surface condition utilizing a heated foaming agent. As a result of Hanning's very different purpose and method, he first injects a pressurized fluid into the mold cavity with the internal air cooling being carried out at the end of his process, as opposed to the present invention which injects cold air near the beginning of the molding cycle.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings, which show, for purposes of illustration only, several embodiments in accordance with the present invention.